1. Field of the Invention
The present invention relates to the structure of the end of a coaxial cable used to transmit RF signals.
2. Description of the Related Art
In RF signal transmission, in an interface circuit, for example, as shown in FIG. 1, the circuit substrate 101 on the driver side (for example, the computer on the server side) and the circuit substrate 102 on the receiver side (for example; a number of personal computers) are connected to a coaxial cable 105 via connectors 103 and 104. In this case, on the transmission route including the circuit substrate 101 on the driver side and the coaxial cable 105, it is ideal that the output impedance of the circuit device 106 on the driver side, the impedance of the pattern part 107 of the circuit substrate, the impedance of a connector 103, the impedance of the coaxial cable 105, the impedance of the processed end part 108 and the like all are the same. This also applies to the receiver side.
This is because if the impedances of such parts differ, a transmission signal is reflected at a part where two impedances are not matched causing its waveform to be distorted etc. Therefore, it is necessary to match impedances, especially in RF signal transmission (for example, the transmission of a signal of 1 GHz or more).
However, FIG. 2 discloses prior art in which the fluctuations of the characteristic impedance of the processed end part of a coaxial cable 117 are suppressed and the impedance is matched with the characteristic impedance of the circuit substrate. In this coaxial cable 117, an inner conductor 112 projected from the tip of an insulator 111 constitutes a signal terminal, and an outer conductor 115 is connected to a grounding terminal 113 via a drain line 116. The grounding terminal 113 is exposed at the end part 114a of an incorporated and molded cover material. Furthermore, the grounding terminal 113 and a circuit substrate, which is not shown in FIG. 2, are connected by pressing the grounding terminal 113 against the grounding electrode of the circuit substrate (for example, see Patent reference 1).
Another prior art reference, FIG. 3 discloses a coaxial cable 121 used for a semiconductor device inspection device. This semiconductor device inspection device comprises a probe core wire 122 at the tip of the coaxial cable 121, for touching the electrode of a semiconductor device, which is not shown in FIG. 3, and transmitting/receiving an inspection signal, and an insulating tube 123 through which the probe core wire 122 is inserted and can be moved. The probe core wire 122 is covered by an insulator 124 and its circumference is covered with an outer conductor (shielding material) 125. Between the outer conductor 125 and insulating tube 123, there is a fixed substrate 126 and a conductor 127. Furthermore, the semiconductor device inspection device comprises a spring for pressing this coaxial cable 121 in such a way that the probe core wire 122 can touch the electrode of the semiconductor, which is not shown in FIG. 3 (for example, see Patent reference 2).
Patent reference 1: Japanese Patent No. 2002-203618
Patent reference 2: Japanese Patent No. S63-317784
However in the technology set forth in Patent reference 1, since the exposed insulator 111 of the processed end part is not shielded by an outer conductor 115, the characteristic impedance of the coaxial cable 117 increases and varies. Because the characteristic impedance (Z) of a coaxial cable generally depends on the inductance (L) and the capacitance (C) per unit length of the cable (Z≅√L/C), a prescribed characteristic impedance value cannot be obtained if the insulator 111 of the processed end part is not shielded by the outer conductor 115.
In the technology set forth in Patent reference 2, since the specific structure of the processed end part is not clear and the exposed insulator 124 is not shielded although the insulating tube 123 and outer conductor 125 are grounded when pressing the insulating tube 123 to the grounding electrode of the circuit substrate, the impedance cannot be improved.